Highly halogenated organic chemicals are favored in industry due to their many useful properties, such as stability under heat and pressure. However, these chemicals are sometimes toxic to flora and fauna. Improper disposal or spills of these organic chemicals may contaminate the environment. Cleanup is necessary due to the considerable health hazard and environmental stability of these chemicals.
In the past, an acceptable procedure for cleaning up a contaminated area involved removal of the contaminated soil or material to a designated secure landfill. Due to recent and upcoming federal regulations, the types and amounts of organic materials that can be disposed of in such a designated landfill have been greatly reduced. Therefore, a growing need exists for an efficient and economical treatment process to treat soils.
The generally accepted treatment technology for destroying highly halogenated organic contaminants is incineration. Application of incineration to soil treatment is inefficient because the contaminants to be incinerated are adhered to a large mass of inert material. Particularly in treating small quantities of soil (&lt;5000 cubic yards), incineration is inefficient because it involves collecting, packaging and transporting the contaminated material to a licensed incineration facility, heating the mass of inert solids to very high incineration temperatures to decompose the proportionately small amount of target contaminants, and packaging and returning the materials back to the site from where they were removed or disposing the materials in a secure landfill. In addition to the labor, transportation and energy costs, there is also a problem given that the capacity of presently licensed incineration facilities is limited.
A similar process to incineration for the disposal or cleanup of contaminated wastes is pyrolysis. Pyrolysis is conducted in a rotating dryer at operating temperatures on the order of 1,500.degree. to 4,000.degree. F. As with incineration, high energy costs result from the elevated temperatures.
As an alternative to incineration and pyrolysis, chemical processes were developed in order to decontaminate soil containing PCBs and chlorinated dibenzodioxins and dibenzofurans. These processes basically involve the treatment of contaminated soil with a dehalogenating agent. A typical reaction scheme involves concurrently reacting an alkali metal hydroxide with an alcohol to form an alkoxide and water; then reacting the alkoxide with the unwanted halogenated aromatic contaminant to form an ether and a salt.
In such a chemical process, the presence of water interferes with the chemical reaction scheme. Thus, the contaminated soil is preferably dried prior to the reactions. After the water has been removed, the dry, contaminated soil is treated with the reagent and the chemical reactions are carried out in a basically sealed system. To accelerate the reaction, the contaminated soil may be mixed with the reagent in an agitated vessel, possibly at an elevated temperature. The chemical treatment techniques are slow and may take weeks if not accelerated by elevated temperature, and involve the expense of spent chemicals.
Furthermore, in cases where relatively small amounts of contaminants are adsorbed to large amounts of inert materials, such as soil or sludge, each of the above techniques involves considerable expense and inconvenience. Accordingly, a keen need has been felt for a more efficient, economical system and apparatus for separating contaminants from contaminated soil, sludge and other inert materials. This need is especially evident where only small amounts (approximately 200-2,000 cubic yards) of contaminated soil or sludge need to be treated. In such situations, there is a need for a system that is adaptable to being highly transportable and cost effective.
Recently, this problem was addressed and methods to thermally decontaminate soil at temperatures below the incineration temperature were developed. Examples of these thermal treatment methods are disclosed in U.S. Pat. No. 4,997,839 (Fochtmann) and U.S. Pat. No. 4,738,206 (Noland). In these thermal processes, the contaminated soil is heated to a temperature sufficient to volatilize the contaminants which are then continuously removed from the heating chamber. Both Fochtmann and Noland disclose the fact that this process should be carried out under a slight negative pressure to avoid fugitive emissions of the volatilized contaminants. Noland also discloses that this slight negative pressure may enhance vapor stripping. However, none of the prior art known to the applicant discloses the use of a strong or high vacuum in conjunction with a thermal decontamination process as is disclosed by the current application. The instant invention utilizes a vacuum of up to about three hundred times as strong as the negative pressure disclosed in either Noland or Fochtmann. Such a high vacuum allows the process to efficiently remove contaminants at a lower temperature than processes operating just below atmospheric pressure (slight vacuum). The lower operating temperature greatly reduces energy consumption. Vent emissions are small, thus minimizing the size of air pollution control equipment. Transport of the system to the contaminated site can therefore be accomplished relatively easily.